Photosynthesis Converting light energy into chemical energy

Photosynthesis

6CO2 + 12H2O + light energy

C6H12O6 + 6O2 + 6H2O

Summary Formula: 6CO2 + 6H2O

Green Energy

C6H12O6 + 6O2

Photoautotrophs capture sunlight and convert it to chemical energy

! Photosynthesis is carried out by • Cyanobacteria • 7 groups of algae • All land plants One mature maple tree makes about 2 tons of sugar per season

Cutaway view of a chloroplast

Leaf Cross Section

Outer membrane

Leaf

Inner membrane

Mesophyll

Vein

• In plants, photosynthesis occurs in chloroplasts. • Mesophyll cells contain chloroplasts.

CO2 O2 Stoma Mesophyll Cell

Thylakoids • Sites of lightdependent reaction

Stroma (space around thylakoids) • light-independent reactions

Granum

Chloroplast Stromal lamellae Thylakoid Thylakoid lumen membrane

Sunlight

Where does the oxygen come from?

Photosynthesis and

The light reaction occurs in the Thylakoids.

Light-independent reactions

Light-dependent reactions

and

The light-independent Calvin cycle occurs in the Stroma.

Sugars

Carbohydrates and other organic substances

Water is split during the light reaction

What is Light?

Light

! Light comes as electromagnetic waves. Visible light ranges from wavelengths of about 400 nm (violet) to 700 nm (red) ! The shorter the wavelength, the greater the energy

! Light is a form of electromagnetic radiation ! What colors do plants absorb?

! Photons are discreet units of energy carried in light

Engelmann Experiment 1880: Action Spectrum of light used in photosynthesis by a green algae Light

A glass prism breaks up a beam of light into a spectrum of colors, which are cast across a microscope slide.

Bacteria Strand of Spirogyra

Chlorophyll Absorption Spectrum ! Chlorophyll absorbs blue and red wavelengths most strongly

! Chlorophyll a absorbs best at 425 & 680 nm ! Chlorophyll b absorbs at 460 & 645 nm ! Carotenoids absorb best at 450 & 490 nm

Leaves are green because we see the reflected -not the absorbed- light

Action Spectrum ! Chlorophylls and carotenoids work together to absorb photons during photosynthesis

a. Chlorophyll structure

Chlorophyll ! Chlorophylls are the major photosynthetic pigments in plants, green algae, and cyanobacteria

in chlorophyll a in chlorophyll b

Lightabsorbing head

Chlorophyll ! Chlorophyll is responsible for absorbing photons (capturing solar power), causing release of electrons – The electrons jump to a higher energy level—the excited state—where electrons are unstable – The electrons drop back down to their “ground state,” and, as they do, release their excess energy

Hydrophobic side chain

Photosystems capture solar power

e–

ATP

! The energy released could be lost as heat or light, but rather it is conserved as it is passed from one molecule to another molecule

e–

e–

e–

– All of the components to accomplish this are organized in thylakoid membranes in clusters called photosystems

NADPH

e– e– Photon

Mill makes ATP

Photon

e–

Two photosystems generate ATP and NADPH ! NADPH, ATP, and O2 are the products of the light reactions ! ATP synthase couples the flow of H+ to the phosphorylation of ADP

Photosystem II

Photosystem I

Light Reactions Chloroplast

Stroma (low H+ concentration) Light

H+

Light

H+

NADP+ + H+

ADP + P

NADPH

H+

ATP

H+

Thylakoid membrane

H2O

1 ! 2

O2 + 2 H+

H+

Photosystem II Electron transport chain Thylakoid space (high H+ concentration)

H+

H+ H+

H+

H+

Photosystem I

H+

+

H

H+

H+

ATP synthase

ATP and NADPH power sugar synthesis in the Calvin cycle Thylakoid membrane

Light

Light reactions

•

! The Calvin cycle makes sugar within a chloroplast Calvin cycle

Sugars

Calvin Cycle Summary Input

CO2 ATP NADPH

CALVIN CYCLE

Output:

G3P

– Requires ATP, and NADPH, which were generated in the light reactions – Requires CO2 – Using these three ingredients, an energy-rich, three-carbon sugar called glyceraldehyde-3phosphate (G3P) is produced – A plant cell may then use G3P to make glucose and other organic molecules

Calvin Cycle ! Carbon fixation – CO2 added to RuBP by Rubisco to produce two 3PGA molecules ! Reduction – NADPH and ATP used to convert 3PGA into G3P, a higher energy molecule used to build sugars ! Regeneration – remaining G3P molecules are used to recreate the starting material RuBP

Rubisco

Photosynthesis uses light energy, CO2 and H2O to make food molecules

Photosynthesis Summary CO2

H2O

! The chloroplast, which integrates the two stages of photosynthesis, makes sugar from CO2

Chloroplast

Light

NADP+

– All but a few microscopic organisms depend on the food-making machinery of photosynthesis – Plants make more food than they actually need and stockpile it as starch in roots, tubers, and fruits

ADP + P Photosystem II

Thylakoid membranes

RuBP

CALVIN CYCLE 3-PGA (in stroma)

Electron transport chains Photosystem I

ATP NADPH

Stroma G3P

O2

Sugars

LIGHT REACTIONS

CALVIN CYCLE

Cellular respiration Cellulose Starch Other organic compounds

Photosynthesis and Cellular Respiration both occur in Plant Cells

Copyright © 2009 Pearson Education, Inc.

In hot and dry climates, plants convert to photorespiration

PEP carboxylase

C4 Plants

C4 Plants CO2

Mesophyll cell

4-C compound

CO2

CALVIN CYCLE Rubisco Sugarcane

Bundle-sheath cell

3-C sugar

• Corn, sugarcane, sorghum, and a number of other grasses • Initially fix carbon using PEP carboxylase in mesophyll cells • Produces oxaloacetate, converted to malate, transported to bundle-sheath cells • Within the bundle-sheath cells, malate is decarboxylated to produce pyruvate and CO2 • Carbon fixation then by rubisco and the Calvin cycle

C4 plant

CAM Plants

CAM Plants • Many succulent (water-storing) plants, such as cacti, pineapples, and some members of about two dozen other plant groups • Stomata open during the night and close during the day – Reverse of that in most plants

• Fix CO2 using PEP carboxylase during the night and store in vacuole

Adaptations for Photosynthesis in hot, dry climates

– Excess CO2 in the atmosphere • Is contributing to global warming

• C3 – Plants that fix carbon using only C3 photosynthesis (the Calvin cycle)

• C4 and CAM – – – – –

Add CO2 to PEP to form 4 carbon molecule Use PEP carboxylase Greater affinity for CO2, no oxidase activity C4 –two pathways occur in different cells CAM – C4 pathway at night and the C3 pathway during the day

Some heat energy escapes into space

Sunlight ATMOSPHERE

Radiant heat trapped by CO2 and other gases